It is known that commercially important polyamides, such as nylon-6, have excellent physical properties in many respects. However, for certain textile applications fabrics and similar products prepared from such nylons are somewhat deficient in moisture absorption as compared to a natural fiber such as cotton. This characteristic is important because according to ENCYCLOPEDIA OF POLYMER SCIENCE, Vol. 10, Section Polyamide Fibers, moisture absorption determines comfort factors, ease and cost of dyeing, antistatic character and hand or feel of the fabric. To overcome this moisture absorption deficiency many attempts have been made, but none have been commercially successful to date.
Disclosed herein is a novel block copolymer which can be converted into a fiber having moisture absorption properties superior to that of commercially used polyamides such as nylon-6. This block copolymer consists of a certain polyamide and a certain poly(dioxa-arylamide). Surprisingly, the incorporation of a certain poly(dioxa-arylamide) into a certain polyamide does not adversely effect the many desirable fiber properties of the polyamide and, in fact, improvement in certain mechanical properties such as initial modulus and strength can be obtained. Furthermore incorporation of said poly(arylamide) materially improves its moisture absorption property. Also the copolymer can be formed into a desired shape by extrusion, injection molding and other well-known thermoplastic forming methods.
A block copolymer can result when a mixture of polymer Y and polymer Z, both of which contain amides, is properly processed. Thus the resulting block copolymer contains relatively long chains of a particular chemical composition, the chains being separated by a polymer of different chemical composition; thus diagrammatically ##STR1## Another type of a block copolymer is one which contains relatively long chains of a particular chemical composition which are separated by a low molecular weight "coupling group", thus diagrammatically ##STR2## Each of the aforementioned polymer chains, i.e., Y and/or Z, can be a homopolymer or a random copolymer.
Generally, copolymers containing an amide functional group, i.e., ##STR3## can be formed by melting two polyamides. Thus, when two different polyamides are mixed and heated above their melting point copolymers are formed. This process is also known as melt blending. However, the length of time the polymers are maintained at a temperature above their melting points has a profound effect on the resulting structure. As the mixing at the elevated temperature begins the mass is a physical mixture of two different compounds. But gradually as the heating and mixing continues the mixture is converted into a copolymer characterized as a "block" copolymer. However, if the heating and mixing continues the length of the "blocks" decrease and sequences of "random" copolymers appear. If the heating and mixing occurs for a sufficient time most of the "blocks" disappear and mostly random sequences form as evidenced by deterioration of its physical properties including melting point. At present there is no known direct way of determining chain sequence of such a polymer. But indirect methods exist and are known to those skilled in the art. Controlled decomposition of such a copolymer will yield all identifiable components that make up the copolymer but will not indicate sequences.
Polymers, including copolymers, containing amide functional groups generally result from a reaction known as condensation. Condensation refers to a polymer forming reaction in which water is a by-product. The various types of polymers that can be produced from condensation (or step growth polymerization) are described hereinafter. The initial stage of a condensation polymerization consists of random combinations of two monomeric units to form dimer molecules. Examples of these could be the formation of two units of nylon-11 from the corresponding amino acid (11-aminoundecanoic acid) in the case of an AB polyamide ##STR4## or adipic acid molecule and hexamethylene diamine in an AABB system ##STR5## The letter "A" refers to one of the functional groups of the monomer, "B" refers to the other.
The foregoing dimer molecules will combine with equal facility with another monomeric unit or a dimer unit. In this fashion, the average degree of polymerization (DB) builds during the course of the reaction. This is discussed in greater detail in ORGANIC CHEMISTRY OF SYNTHETIC HIGH POLYMERS, Robert W. Lenz, Library of Congress Catalog Card No. 66-22057.
In the same manner, as reactions I and II, random copolymers can be formed. The only condition necessary is that more than one type (or two if an AABB system is used) of monomer unit be present during the condensation reaction. Thus following from the example above where monomers of AB and AABB polymers are present in the same reactor at the beginning of the polymerization, the AB monomer (amino acid) will react with a similar unit or the AABB monomer unit (the diamine or diacid) in a random fashion since their reactivities are similar. The final result of such a polymerization will be a random copolymer. If their reactivities are very dissimilar, there would be a tendency to become blocks, however, units having similar carboylic and/or similar amine ends have similar reactivities. Further examples of random copolymers are given in U.S. Pat. No. 3,397,107, where the monomer units of nylon 303/T and caprolactam are polymerized in a random fashion. Another example is contained in U.S. Pat. No. 3,594,266, in which a polyethylene oxide diamine, terephthalic acid and caprolactam were polymerized in a random fashion. Since the condensation polymerization is a random sequence of events it would be extremely improbable to obtain an alternating copolymer using dissimilar monomer units in the condensation reaction as it is known today. An alternating copolymer can be classified as a special type of random copolymer.
Formation of a condensation block copolymer cannot be easily achieved using the conditions described heretofore because of the random reaction of monomeric units. Block copolymer preparations have been described in the patent literature using at least two techniques. One technique, as described before, is melt blending two homopolymers at temperatures where the polyamide becomes reactive to imide interchange, chain extension and hydrolysis. Such a technique is disclosed in U.S. Pat. No. 3,393,252. When the conditions are closely controlled block copolymers with a distribution of optimum lengths can be prepared.
Another method of preparing block copolymers is described in U.S. Pat. No. 3,683,047. It consists of polymerizing two homoprepolymers of low molecular weight from such as 1000 to 4000. In this specific case, one prepolymer was carboxyl terminated while the other was amine terminated. The result of the polymerization is a block copolymer. Under the conditions of polymerization very little randomization occurred as indicated by little loss in melting point during the blend time. These block copolymers have been called ordered copolymers since by the nature of the starting materials reactive functional groups cannot react with themselves.
Because of the complexity in naming the copolymers of polyamide and poly(dioxa-amide), a shorthand nomenclature is used herein. It is based in part on the nomenclature used to identify aliphatic polyamides. Numbers signify the number of carbon atoms in a polymer. The letter "O" signifies oxygen and its relative location within the polymer; "N" signifies polyamide linkage; "T" signifies terephthalic. Thus "30203" refers to a diamine function while "6" refers to the diacid function. Therefore, "6" refers to six carbon paraffinic diacid and in particular adipic acid. Also "30203" indicates the number of paraffinic carbons and the "O" indicates the placement of oxygen. In this nomenclature a slash (/) designates a random copolymer whereas a double slash (//) indicates a block copolymer. Thus N-30203-6//6 indicates that blocks of N-30203-6 are connected within the copolymer with blocks of "6" (nylon-6).
Contrary to expectations based on the previously discussed art it has now been found that it is possible to prepare a composition comprising a block copolymer of polyamide and poly(oxaamide) having a moisture uptake better than that of its polyamide precursor, e.g. nylon-6. In addition fibers of the copolymer have overall fiber properties substantially equivalent to that of such nylons as nylon-6.